Today, one major problem when manufacturing industrial laser bars is the large mismatch in thermal expansion coefficient (CTE) between the commonly used laser diodes and the cooler. For example, GaAs-based laser diode bars have a CTE=6.5×10−6 K−1, whereas the usual copper cooler has a CTE=16×10−6 K−1.
There are three common mounting technologies for industrial laser bars on copper coolers:
(1) The laser bar is directly attached to the copper cooler using a “soft solder”, e.g. In, InAg, or InSn.
(2) The laser bar is attached to a “CTE-matched” CuW submount, consisting e.g. of a homogenized 10% Cu and 90% W submount, forming a bar-on-submount structure (BoS), using a “hard solder”, e.g. AuSn, and then                (2a) mounting the BoS on the copper cooler using a “soft solder”, e.g. In, InAg, or InSn, or        (2b) mounting the BoS on the cooper cooler using a “hard solder”, e.g. AuSn, SnAgCu, or PbSn.        
For the following reasons, none of these three mounting technologies results in a satisfactory assembly for industrial laser bars:
One reason is the insufficient stability of the solder interface which results in an unsatisfactory reliability. A drawback of “soft” (i.e. low melting point) solders is their instability under thermal cycling operation, e.g. on-off operation common in industrial laser applications. As a consequence, with the mounting technologies described in (1) and (2a) above, the limiting operating condition is not determined by the properties of the laser diodes, but by the poor stability of the solder interfaces. Tests have shown that for one particular diode design, the maximum drive current for a reliable operation is about 90 A when using the mounting technology (1), i.e. direct mounting the diode onto the copper cooler using In. For the technology (2a), the maximum drive current is 120 A, i.e. mounting the BoS on the copper cooler using InAg. When using hard solder only as described in (2b), it is 180 A. As a consequence, “soft solder” technologies seem to be no option for future industrial laser bar generations to meet the market requirement of a very high optical output power.
For the temperature-induced deformation of a laser bar caused by the temperature difference between the mounting condition and the use condition on or with its mount or submount, persons skilled in the art use the term “smile” as a descriptor because of its appearance. “Smile” of a laser device in this context is defined as the warping or curvature or bow of a laser device along the length of the laser diode bar which is in the plane orthogonal to the emitted light beam, i.e. orthogonal to the emitted light beam. Thus, looking head-on into the light-emitting facets of the laser diodes of the bar, the various facets do not form a straight line. Smile is generally believed to result from stress and the term is often used to imply that the device has been subject to thermal stress.
Because technology (1) avoids a submount, it allows the design of devices with better thermal conductivity than comparable devices using the technologies (2a) and (2b). Also, because of the low solder temperature and the ductility of the soft solder, devices assembled using this technology have low bow values, i.e. <2 μm. Further, vertically stacked laser bar arrays for very high power output may be made smaller, thus enabling better and easier vertical collimation of the laser beam by lenses or other optical means.
However, as mentioned above, the limited reliability of soft-soldered devices in off-on operation is an important drawback of this technology.
Technology (2a) uses a submount which is CTE-matched to the laser bar and a ductile soft solder between the various parts. This results in low-bow and low-stress devices. Further, such devices are significantly more reliable than comparable devices assembled with technology (1). This behavior is based on the fact that, because of the missing submount in technology (1), the soft In-based solder interface is close to the light/heat-generating region responsible for thermal and thermo-mechanical driving forces, which, for an on-off operation mode, cause a degradation of soft-solder interfaces. These driving forces are directly correlated to the spatio-temporal temperature distribution in the solder interface. Because of the thermal spreading within the submount, the temperature distribution is more homogeneous for technology (2a) than for technology (1), where there is no submount acting as a heat spreader between the heat-generating region and the soft solder interface. Nevertheless, the maximum reliable operation power of devices assembled using technology (2a) is in many cases determined by the stability of the soft solder interface. “Hard solder” assembly technologies avoid this problem and achieve more reliable operation of high power devices.
Technology (2b) offers such a pure hard solder assembly. The CuW submount, having a thermal expansion coefficient (CTEsub) equal or close to the thermal expansion coefficient (CTEbar) of the laser bar, acts as a stress buffer between the copper cooler and the laser bar. Nevertheless, the limitation of the thermal expansion coefficient to a narrow region centered at 6.5×10−6K−1 often leads to non-optimized device characteristics, such as high smile values, undefined spectral shape or poor polarization purity.
Further, stress within and the often resulting smile of a laser device have a significant impact on the reliability. For some devices, e.g. devices having a stress-sensitive epitaxial structure, technology (2b) often leads to reliability problems, because e.g. a hard solder and a CuW submount are unable to compensate the uncontrolled compressive stress in the device caused by the thermal mismatch between the laser/submount and the cooler.
Also, to control the CTE-mismatch between diode and cooler, so-called CTE-matched coolers have been developed. Known technologies for CTE-matched coolers are:                CuMoCu micro channel coolers;        Cu—AlN micro channel coolers; and        Al—C (nanotubes) passive coolers.        
Although these coolers are technically quite advanced, they have some disadvantages:                they are expensive and are therefore used only for demonstration or “niche” applications;        some users expect cooler reliability problems and therefore hesitate to switch to a CTE-matched cooler; and/or        the thermal conductivity of the CTE-matched coolers is in general not as good as the thermal conductivity of a copper cooler with the same geometry.        
Also, layered submounts have been developed to obtain a better match between the laser diode bar and the cooler, i.e. these layered submounts aim to match the CTEbar of the laser bar to reduce the stress to the latter. This may seem to solve the problem but it does not. The reason is that the application of heat when soldering the laser bar to the layered submount inevitably introduces uncontrolled stress.
A specific example of a high power laser mounting module discloses Haden in U.S. Pat. No. 5,848,083. This module includes a bulk layer with stress-relief apertures and consists of at least two components: a mounting plate and a multi-layer heat transfer component. The cooler is mounted to the mounting plate. Apart from being rather complex and thus costly, the main object of Haden's design is to produce an “expansion-matched, stress-relieved” module with high thermal conductivity. The CTE of this design is said to be “substantially equal” to the CTE of the “heat dissipating element”, i.e. the laser bar. In other words, Haden discloses an approach equivalent to the type (2b) technology described above.
However, all prior art solutions are focused on the avoidance or minimization of stress in the laser bar and address potential solutions therefore. To summarize, despite the various partial solutions for the stress problem of laser diode bar devices, there is still a need for a simple, cost-effective design of such devices.